Understanding autism's biological diversity could transform how clinicians approach diagnosis and treatment selection. For decades, researchers have suspected that autism's wide-ranging symptoms reflect distinct underlying brain mechanisms, but identifying these biological subtypes has remained elusive. This breakthrough analysis of nearly 2,000 individuals provides the first clear evidence that autism spectrum disorder comprises at least two neurobiologically distinct conditions. Using advanced brain imaging across both human patients and 20 genetic mouse models, investigators discovered that autism separates into hypoconnectivity-dominant and hyperconnectivity-dominant subtypes based on how brain regions communicate. The hypoconnectivity subtype shows weakened neural connections and links to synaptic dysfunction—the molecular machinery where brain cells communicate. The hyperconnectivity subtype exhibits overactive brain networks associated with transcriptional regulation and immune system dysregulation. These patterns proved remarkably consistent across species, with mouse models showing identical connectivity signatures that mapped to the same biological pathways found in humans. The human subtypes demonstrated distinct behavioral profiles and were highly replicable across multiple research centers, suggesting robust biological validity rather than statistical artifact. This cross-species validation represents a significant methodological advance, as findings that replicate from engineered mouse models to human populations carry substantially greater confidence than human-only studies. The work challenges the current one-size-fits-all approach to autism research and treatment. Rather than viewing autism as a single disorder with variable presentation, these findings suggest clinicians may eventually stratify patients into connectivity-based subtypes for more targeted interventions. However, translating these research findings into clinical practice will require additional validation studies and development of practical diagnostic tools that can reliably identify these subtypes in clinical settings.